1. Technical Field
The present disclosure relates to frequency circuits and, more specifically, to a system and method for enhancing re-locking of a phase locked loop (PLL) system, i.e., through this methodology the PLL will be locked almost instantly.
2. Description of the Related Art
In the field of frequency synthesis, phase lock loops (PLLs) are employed to act as wide frequency range tracking filters and as wide frequency range frequency translators. When thus used, the phase lock loop is designed to lock a synthesized signal of a first oscillator at a first frequency and at a first phase to the phase of an incoming reference signal at another frequency. Once the frequency of the synthesized signal developed in the phase lock loop (PLL) is matched to that of the incoming reference signal, steady state is said to exist. However, when the PLL is powered down and then switched on, the phase lock loop experiences a transient state before the loop locks on and settles to the correct reference frequency. For many applications, where the PLL is switched on and off in the same configuration frequently, it is important to reduce the time spent in this transient state.
U.S. Pat. No. 4,937,536 discloses a phase lock loop frequency synthesizer for providing a synthesized frequency signal employing a modified adaptive loop construction having parallel feedback paths about a loop amplifier. A normal feedback path having a narrow bandwidth characteristic includes a feedback capacitor having one end connected to electrical ground via a controlled switch and a second feedback path having a wide bandwidth characteristic with a capacitor also connected across the amplifier. Upon the variation of an incoming reference signal, the controlled switch connects the normal feedback capacitor to ground permitting the wide bandwidth feedback path to rapidly settle the loop while charging the feedback capacitor of the normal feedback path. Upon opening the controlled switch, the narrow bandwidth feedback path completes the charging of the feedback capacitor of the normal feedback loop settling the loop to the steady state condition while enhancing the settling time.
U.S. Pat. No. 4,559,505 discloses a frequency synthesizer is provided including a reference frequency generator coupled to one input of a phase detector. The output of the phase detector is coupled via a pair of alternately connected filters through a voltage controlled oscillator and a divider circuit to the remaining input of the phase detector to form a phase locked loop. The first filter of the pair is designated for operation on a main channel frequency while the remaining filter is designated for operation on a priority channel frequency. The capacitive elements of each respective filter remain fully charged up for operation on their respective frequencies and thus when such filters are alternately switched between to change frequency from the main channel to the priority channel, the capacitive elements need not be charged to new levels to accommodate such frequency change. Thus, switching between a main channel and a priority channel is accomplished in a minimal amount of time with a significant reduction in frequency synthesizer energy requirements.
The patents as described above lack in providing a fast, reliable and efficient mechanism for the fast relocking of the PLL. In the first patent the reduction in lock time is small enough. Whereas the second patent uses multiple loop filters for a fast frequency acquisition, but this methodology causes an increased PLL area.
Therefore, there is a need for a novel methodology that can improve re-locking time of a PLL system by locking the PLL instantaneously.